DETERMINATION OF RESERVOIR PROPERTIES AT WAIRAKEI GEOTHERMAL-FIELD USING GRAVITY CHANGE MEASUREMENTS

Exploitation of a liquid-dominated geothermal system generally results in the transfer of mass within, or out of, the system and causes meas urable changes in gravity. When the rate of mass transfer is controlle d by the permeability of the reservoir rocks, then the analysis of mea sured gravity changes can yield values for reservoir properties. For t wo such cases we derive permeability or permeability-thickness (transm issivity) and storativity. One case is during the early stages of expl oitation when a two-phase zone is rapidly expanding. Calculations usin g a numerical reservoir simulation model show that for Wairakei the gr avity changes associated with permeabilities of 50 and 100 md would be clearly distinguishable (> 50 muGal) in less than 2 years. A measured gravity change of -415 muGal between 1950 and 1961 suggests a permeab ility of 100 md, which is consistent with values obtained from well te sts. The second case is during reinjection into a deep-liquid zone ove rlain by a two-phase zone. Gravity changes of up to 120 muGal associat ed with reinjection at Wairakei are analysed using analytical models ( Theis solution). The measured gravity changes are matched by those of a model with isotropic permeability having a permeability-thickness (k h) of 9.9 d-m and a storativity of 9.2 x 10(-6) m Pa-1. If a model hav ing anisotropic (horizontal) permeability is used the derived values f or kh are 18.2 and 5.4 d-m, and the storativity is 8.7 x 10(-6) m Pa-1 .